A 2D Coordination Network That Detects Nitro Explosives in Water

Jul 21, 2017 - The solvothermal reaction of Zn(NO3)2·6H2O and a linear dicarboxylate ligand H2L, in the presence of urotropine in N,N′-dimethylform...
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A 2D Coordination Network That Detects Nitro Explosives in Water, Catalyzes Baylis−Hillman Reactions, and Undergoes Unusual 2D→3D Single-Crystal to Single-Crystal Transformation Vivekanand Sharma, Dinesh De, Sanchari Pal, Prithwidip Saha, and Parimal K. Bharadwaj* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India S Supporting Information *

ABSTRACT: The solvothermal reaction of Zn(NO3)2·6H2O and a linear dicarboxylate ligand H2L, in the presence of urotropine in N,N′-dimethylformamide (DMF), gives rise to a new porous two-dimensional (2D) coordination network, {[Zn3(L)3(urotropine)2]·2DMF·3H2O}n (1), with hxl topology. Interestingly, framework 1 exhibits excellent emission properties owing to the presence of naphthalene moiety in the linker H2L, that can be efficiently suppressed by subtle quantity of nitro explosives in aqueous medium. Furthermore, presence of urotropine molecules bound to the metal centers, 1 is found to be an excellent heterogeneous catalyst meant for atom-economical C−C bond-forming Baylis− Hillman reactions. Additionally, crystals of 1 undergo complete transmetalation with Cu(II) to afford isostructural 1Cu. Moreover, the 2D framework of 1 allows replacement of urotropine molecules by 4,4′-azopyridine (azp) linker resulting in a three-dimensional (3D) metal−organic framework, {[Zn(L)(azp)]·4DMF 2H2O}n (2). The 1→2 transformation takes place in single-crystal-to-single crystal manner supported by powder X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy, and morphological studies. Remarkably, during this 2D→3D transformation, the original trinuclear [Zn3(COO)6] secondary building unit changes to a mononuclear node, which is unprecedented.

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INTRODUCTION Coordination polymers (CPs) continue to be a subject of considerable current interest, as they provide new chemistry in addition to their versatile applications in several contemporary areas of materials research.1−4 In constructing a CP, both the metal ions as well as the linkers play vital roles in terms of directing the overall structure besides the nature and sizes of the voids. Particularly, in a two-dimensional (2D) coordination network the guest molecules can converge due to favorable host−guest interactions leading to catalysis, sensory applications, and so on.5−9 The linker H2L has been designed keeping the aforesaid aspects in mind. It forms a 2D framework, {[Zn3(L)3(urotropine)2]·2DMF·3H2O}n (1) (DMF = dimethylformamide) in the presence of urotropine (Scheme 1). The

free N atoms of urotropine in 1 could be used to explore the Baylis−Hillman (BH) reaction of an aldehyde and an α,βunsaturated carbonyl compound to give α-methylene-βhydroxy carbonyl derivative. This atom-economical reaction has been proven a highly versatile synthetic method for the C− C bond formation.10 Presence of a naphthalene moiety in the linker makes the framework 1 highly emissive in nature allowing to probe its sensing abilities toward nitroaromatic compounds (NACs) that are explosives and responsible for environmental pollution.11,12 The axially bound urotropine is also amenable to probe if it can be replaced by a stronger ligand with donor atoms at both ends leading to the conversion from a 2D to a threedimensional (3D) framework. In this context, it is relevant to point out that flexible and dynamic CPs that undergo singlecrystal to single-crystal (SC−SC) transformations involving allied movements of atoms has emerged as a fascinating solidstate phenomenon.13−23 This can potentially augment its functionalities and also allows to arrive at a structure that may not be accessible by the de novo synthesis. Indeed, compound 1 undergoes 2D→3D transformation via replacement of the urotropine by the 4,4′-azopyridine (azp) colinker (Scheme 1) to afford 2 in SC−SC manner. Quite interestingly, this transformation takes place with the trinuclear [Zn3(COO)6]

Scheme 1. Schematic Diagrams of the Ligands

Received: March 25, 2017 Published: July 21, 2017 © 2017 American Chemical Society

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Inorganic Chemistry Scheme 2. Synthetic Route for H2L

Figure 1. (a) Coordination environment of Zn(II) ions in 1; symmetry codes used to generate equivalent atoms: (i) 1 − x + y, 1 − x, z; (ii) 1 − y, x − y, z, (b) crystal structure of 1, (c) 2D-layered structure in 1 along the c-axis, (d) packing of 1 in an ABA···fashion, (e) topological views of 1. (H atoms are omitted for clarity). g, 2.36 mmol) in DMF and water (1:1, 50 mL) was heated at 75 °C for 24 h. After that the mixture was cooled to room temperature and extracted with ethyl acetate (EtOAc) three times. The EtOAc part was dried over anhydrous sodium sulfate and evaporated to dryness. The desired ester, methyl 6-(4-(ethoxycarbonyl)phenyl)-2-naphthoate, was separated by column chromatography (eluent, n-hexane/EtOAc = 9:1, v/v), as a white solid (yield: 2.72 g, 86%). 1HNMR (400 MHz, CDCl3, ppm): δ = 8.62 (s, 1H), 8.15 (d, 2H), 8.09 (d, 2H), 8.02 (d, 1H), 7.93 (d, 1H), 7.78 (t, 3H), 4.41 (q, 2H), 3.98 (s, 3H), 1.42 (t, 3H); 13 CNMR (100 MHz, CDCl3, ppm): δ = 166.88, 144.91, 139.81, 135.75, 132.06, 130.88, 130.24, 129.80, 128.63, 127.87, 127.44, 126.18, 126.00. The Second Step. A mixture of the white carboxylic ester (2.72 g, 5 mmol), NaOH (3 g, 75 mmol), 25 mL of tetrahydrofuran (THF), 25 mL of MeOH, and 25 mL of H2O was refluxed for 2 d. After that THF and MeOH were evaporated off. After 50 mL of water was added, the solution was acidified with diluted HCl (1 M) until no further

secondary building unit (SBU) changing to a mononuclear node, which is unprecedented (to the best of our knowledge). However, all attempts to synthesize 2 via solvothermal reactions taking different equivalents of H2L, azp, and Zn(NO3)2 remained unsuccessful and always led to the formation of a different 3D structure, {[Zn2(L)2(azp)]· 8DMF·3H2O}n(3).

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EXPERIMENTAL SECTION

Physical Measurements. All instrumentations and X-ray structural studies are given in the Supporting Information. Synthesis of Ligand H2L. The ligand H2L was synthesized by the following two steps (Scheme 2). The First Step. A mixture of methyl-6-bromo-2-napthoate (2.5 g, 9.4 mmol), 4-(ethoxycarbonyl) phenyl boronic acid (2.19 g, 11.28 mmol), Pd(OAc)2 (0.02 g, 0.089 mmol), and sodium carbonate (0.25 8848

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Inorganic Chemistry precipitation (pH ≈ 2−3). The precipitate was separated by filtration, washed repeatedly with water, and dried under vacuum to get H2L in 90% yield (2.45 g). 1H NMR (400 MHz, deuterated dimethyl sulfoxide (DMSO-d6)): δ = 8.61 (s, 1H), 8.35 (s, 1H), 8.19 (d, 1H), 8.08−8.03 (t, 3H), 7.99 (d, 1H), 7.97−7.93 (m, 3H);13CNMR (400 MHz, DMSO-d6): δ = 167.79, 144.16, 139.06, 135.71, 132.26, 130.67, 129.17, 127.81, 126.32. Electrospray ionization mass spectrometry (ESI-MS): [M-H], m/z: 291.0763 (calcd for C18H12O4, 292.0736). Anal. Calcd for C18H12O4: C, 73.97%; H, 4.14%. Found: C, 74.04; H, 4.2%. Fourier transform infrared (FT-IR; KBr pellets, cm−1): 3452(br, s), 2926 (w), 1632 (s), 1384 (s), 1423 (b), 695 (s), 545 (w). Synthesis of {[Zn3(L)3(urotropine)2]·2DMF·3H2O}n (1). Zn(NO3)2·6H2O (20 mg, 0.0672 mmol), H2L (20 mg, 0.0684 mmol) and urotropine (10 mg, 0.0713 mmol) were dissolved in 3 mL of DMF. It was transferred to a Teflon-lined stainless steel autoclave and heated at 90 °C for 3 d and then cooled to room temperature at the rate of 10 °C h−1. Colorless block-shape crystals of 1 were obtained. Yield ∼65%. FT-IR (KBr pellets, cm−1): 3452 (broad), 2930 (w), 1659 (s), 1544 (w), 1392 (m), 1106 (w), 786 (w); Anal. Calcd (%) for C72H74N10O17Zn3: C 55.88, H 4.82, N 9.05; found: C 55.92, H 4.90, N 9.01. Synthesis of {[Zn(L)(azp)]·4DMF·2H2O}n (2). Colorless crystals of 1 were soaked in a solution of 0.05 M azp in DMF for 3 d at room temperature to obtain morphologically same but structurally different orange-colored crystal of 2. FT-IR (KBr pellets, cm−1): 3445 (broad), 1608 (m), 1384 (m), 784 (w); Anal. Calcd (%) for C40H50N8O10Zn: C 55.33, H 5.80, N 12.91; found: C 54.37, H 5.84, N 12.90. Synthesis of {[Zn2(L)2(azp)]·8DMF·3H2O}n (3). Zn(NO3)2·6H2O (20 mg, 0.0672 mmol), H2L (20 mg, 0.0684 mmol), and azp (10 mg, 0.0543 mmol) were dissolved in 3 mL of DMF. It was then transferred to a Teflon-lined stainless steel autoclave and heated at 90 °C for 3 d and then cooled to room temperature at the rate of 10 °C h−1. Redcolored needle-shaped crystals of 3 were collected by filtration. Yield ≈ 55%. FT-IR (KBr pellets, cm−1): 3446 (broad), 2931 (w), 1657 (s), 1540 (w), 1493 (w), 1418 (s), 1385 (s), 1254 (w); Anal. Calcd (%) for C70H90N12O19Zn2: C 54.80, H 5.91, N 10.95; found: C 54.97, H 5.98, N 10.86. All our attempts to de novo synthesize compound 2 solvothermally by taking different molar ratios of the reactants, H2L, azp, and Zn(NO3)2·6H2O remained unsuccessful.

diffraction (PXRD) pattern of 1 matched closely to the simulated pattern (generated from the CIF file) confirming its bulk phase purity (Figure S11, Supporting Information). The thermogram of 1 (Figure S12, Supporting Information) showed 12.9% weight loss due to the removal of lattice DMF and water molecules up to ∼240 °C. Decomposition of the framework could be achieved only beyond 400 °C signifying its high thermal stability. Catalytic Activities in Baylis−Hillman Reactions. The high crystallinity and accessibility of three N atoms of urotropine in 1 made it attractive for possible use as a heterogeneous catalyst. Because of its structural features, we explored the catalytic activity of 1 for the Baylis−Hillman reaction. This reaction is highly important in organic synthesis being atom-economical and forming specific C−C bond along with generation of functional groups that can be utilized for the stereoselective synthesis of various multifunctional molecules.25−28 This reaction can be performed by using a Lewis base, particularly tertiary amines like DABCO, urotropine, or tertiary phosphine, etc. However, homogeneous catalysts are very often difficult to recover, and/or they decompose at the time of catalysis reaction, impeding their reuse. To overcome these limitations, attempts have been made to immobilize the catalytic part in a solid support. In a recent report, the ketonebased Baylis−Hillman reaction catalyzed by L-proline-functionalized metal−organic framework (MOF) along with imidazole nucleophile has been reported.29 In 1, the urotropine molecules remaining fixed through metal binding provides an ideal opportunity to explore heterogeneous catalysis of the Baylis− Hillman reactions. Here, methyl vinyl ketone (MVK)-based Baylis−Hillman reactions were performed (Table 1). We chose the reaction between MVK and para-nitrobenzaldehyde as a model reaction (Table 1, entry 1). The reaction generated the corresponding product with satisfactory isolated yield (72% in 32 h) after column chromatography. The

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Table 1. Results Obtained for the Baylis−Hillman Reactions Catalysed by 1a

RESULTS AND DISCUSSION The framework 1 crystallized in the trigonal space group R3̅ (Table S1, Supporting Information) as revealed from singlecrystal X-ray diffraction study. The asymmetric unit consisted of three Zn(II) centers (having one-third ocupancy), one L2− ligand, one urotropine, and lattice solvent molecules. These three Zn(II) ions were chelated by six carboxylate groups from six L2− ligands to construct a trinuclear [Zn3(COO)6] SBU. In the SBU, the terminal Zn1 and Zn3 were found to be distorted tetrahedral with ligation from three carboxylate O and one urotropine N donors (Figure 1a). In contrast, the middle Zn2 was hexa-coordinated, with ligation from six carboxylate O adopting an octahedral coordination geometry, and separations between adjacent Zn(II) centers in the SBU were 3.463 and 3.515 Å. The urotropine molecules, bound to each terminal Zn(II) center, were like dangling ligands that controlled the dimensionality (Figure 1b). Each SBU further linked six other SBUs through L2− ligands to construct a 2D layer framework (Figure 1c). The infinite 2D layers pack into an ABA···fashion (Figure 1d). The topology of 1 can be explained as a 6-c uninodal hxl net (Figure 1e). The framework 1 has one-dimensional (1D) rhombic channels of dimension 6.06 × 5.55 Å2 along the b-axis (distance refers to atom-to-atom throughout; Figure S10, Supporting Information). PLATON24 calculations showed the void volume to be 34.6% of the unit cell volume. Powder X-ray

a

Reaction conditions: aldehyde (0.1 mmol) and MVK (0.2 mmol) in 1 mL of solvent as above, catalyst 1 (10 wt %) 25 °C for 32 h. bIsolated yields after silica gel chromatography.

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Figure 2. (a) Emission and excitation profiles of 1 in water suspension, (b) emission profiles of free ligand and that of 1.

Figure 3. (a) Emission profile of 1 in the presence of different analytes; (b) bar diagram to show the increasing quenching efficiencies.

(Figure S26, Supporting Information). Nitrobenzene (NB), 1,4-dinitrobenzene (1,4-DNB), 4-nitrophenol (4-NP), 2,4,6trinitrophenol (TNP), 3-nitrotoluene (3-NT), and 4-nitrotoluene (4-NT) were tested to probe its sensory application. The NACs are electron-deficient in nature; it is, therefore, presumed that they can interact with the electron-rich naphthalene moiety of the framework. Simultaneously, favorable interactions of acidic TNP with the Lewis basic free aliphatic amine groups of urotropine may also contribute to quenching of the emission of 1.31 When 1 suspended in water was excited at 315 nm, it exhibited the emission maximum at 377 nm (Figure 2a), whereas a DMF solution of H2L exhibited the emission maximum at 380 nm when excited at 310 nm. Both the profiles were quite similar and attributable to π→π* transitions of the ligand (Figure 2b).31 For detection purposes, DMF solutions of each analyte were prepared having concentration 10 mM. Analyte solution (1 mL) was mixed with 1 mL of stock solution of 1 in water. The quenching efficiencies (Figure 3) were different for different analytes and were estimated using the equation [(I0 − I)/I0] × 100, where I0 and I being the luminescence intensities of 1 before and after the addition of NAC. The TNP exhibited highest quenching. The quenching percentages (QP) of different NACs are TNP (∼99%), 1,4-nitrophenol (QP ≈ 77%), 1,4-DNB(QP ≈ 58%), 3,-NT(QP ≈ 55%), 4-NT(QP ≈ 46%), and NB(QP ≈ 32%). Changing the dispersion medium for 1 to toluene or mesitylene in place of water had no noticeable effect on its quenching abilities.

substrates 3-nitrobenzaldehyde, 2-nitrobenzaldehyde, and 4fluorobenzaldehyde afforded the corresponding products with satisfactory isolated yields (Table 1, entries 2−4). However, for benzaldehyde, the reaction furnished the moderate yield (34%) of product (entry 5). Similar results were obtained with urotropine as the homogeneous catalyst for the substrate, 4nitrobenzaldehyde (Table 1, entry 6). In addition, the absence of catalyst (entry 7) provided no product in the same time period. The formations of the desired products were confirmed by the 1H NMR and 13C NMR (Figures S13−S22, Supporting Information). Typically, once a reaction was over, compound 1 was collected by filtration and after washing and air-drying could be used in the successive runs. A slight degradation of catalytic activity of 1 was experienced only after four cycles (Figure S23, Supporting Information); nevertheless, the crystallinity was maintained throughout the reaction as proven by PXRD (Figure S24, Supporting Information). Heterogeneous nature of the catalyst 1 was probed by adopting the standard protocol,30 where the whole suspension was filtered after 15 h (∼48% yield) to remove the catalyst. In the filtrate the reaction did not proceed further (Figure S25, Supporting Information) establishing the heterogeneous nature of the catalyst. Sensing of Nitroaromatics. The linker H2L having naphthalene moiety, the framework 1 was highly luminescent in nature. In the 2D framework, these naphthalene moieties were exposed and, therefore, allowed to probe its sensing abilities toward electron-deficient nitroaromatic compounds (NACs) that are explosives in nature. Framework 1 was found to be highly stable in water as verified by the PXRD analysis 8850

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been observed in their corresponding SV plot except for TNP. A deviation from linearity is observed in SV plot of TNP by rising upward with a high quenching constant (KSV ≈ 5.1 × 104 M−1; Figure 5). With increase in concentration, the growth of nonlinear trait specifies that the quenching mechanism is determined by dynamic quenching owing to the collisions of TNP molecules into the channel of 1 plus static quenching mechanism usually observed for such complexes.32−34 The compound 1 could detect the TNP at very lower concentration having a detection limit of 1.93 ppm (Figure S27, Supporting Information). In general, the conduction band (CB) of coordination polymers are resides above the lowest unoccupied molecular orbital (LUMO) of these electron-poor nitroaromatics.31 Photoinduced electron transfer can occur from the conduction band of 1 to LUMO of the nitroaromatics when excited offering to the quenching.35 But merely, such electron transfer phenomena can not be the cause for high of fluorescence quenching. It is a well-known fact that the resonance energy can be transferred when the absorption band of the analytes overlap with the emission band of the fluorophore (here compound 1). The amount of energy transfer depends on the area of overlap. Figure 6a corresponds to considerable overlap of the absorption curve of TNP with the emission curve of 1, slightly with 4-nitrophenol, and almost negligible overlap happens in other analytes. These observations clearly support the highest quenching efficiency by TNP followed by 4-nitrophenol with respect to other nitroaromatics.

The emission spectra of TNP was recorded by adding each analyte (10 mM) to 1 mL of 1. With increasing amount of each analyte, the flourescence intensity of 1 decresed correspondingly. The Stern−Volmer (SV) equation, (I0/I) = KSV[A] + 1, can be used to interpret the quenching efficiency. Here I0 represents the initial intensity of 1 in absence of analyte, I is the intensity in the presence of analyte with molar concentration [A], and KSV is the quenching constant (M−1). The Stern−Volmer plots for all the analytes are shown in Figure 4. A slight linear uplift with increase in concentration has

Figure 4. Stern−Volmer plot for different nitroaromatics.

Figure 5. (a) Emission spectra of 1 upon gradual increase of TNP concentration (10 mM); (b) the resultant Stern−Volmer plot. 8851

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Figure 6. (a) Figure to show the spectral overlap; (b) reversibility test for 1 (analyte TNP).

Figure 7. (a) Coordination environment of Zn(II) ions in 2, symmetry codes used to generate equivalent atoms: (i) −x, y, 0.5 − z, (b) perspective view of 2 along a-axis to show fivefold interpenetration, and (c) topological view of 2.

into a 3D framework having the formula {[Zn(L)(azp)]· 4DMF·2H2O}n (2). The space group changed to monoclinic C2/c along with considerable change in the lattice parameters (Table S1, Supporting Information). Most significantly, the original trinuclear [Zn3(COO)6] SBU present in 1 changed to a mononuclear node with distorted tetrahedral geometry around the metal (Figure 7a). From the topological viewpoint, the framework 2 could be described as a 4-c dia 3D network with fivefold interpenetration (Figure 7b,c). PLATON calculations indicated that 2 had 38.4% void volume (1264 Å3 of the unit cell volume of 3290 Å3). The simulated and experimental PXRD patterns of 2 were well-comparable (Figure S30, Supporting Information). The thermogram of 2 revealed a loss in weight of 37.8% before 300 °C (Figure S31, Supporting Information), corresponding to the release of four DMF and two H2O guest molecules per formula unit (calcd 37.8%). We performed several experiments to determine that 1→2 transformation occurred in SC−SC manner and not via dissolution of 1 followed by growth of 2 in DMF. A good quality crystal of 1 was selected and immersed in a DMF solution of azp, and its change of color was optically monitored every 10 h interval (Figure 9a). Throughout the exchange reaction, no appreciable morphological difference in the crystal between the parent and the daughter framework was observed. For further evidence, atomic force microscopic (AFM) technique was applied. The AFM is an excellent technique to examine the SC−SC processes in a dynamic zone.36−39 We executed the time-dependent study of ultrathin film of 1 in azp

To prove the reversibility of 1, it was dispersed in a DMF solution of trinitrophenol and centrifuged after 1 d. The luminescence intensity of 1 was measured after thoroughly washing with DMF. The luminescence intensity was almost unchanged at least up to three cycles implying the reversible nature of sensing (Figure 6b). SC−SC Transformations. The presence of accessible [Zn3(COO)6] SBUs motivated us to study the metal ion exchange through a transmetalation process. When the crystal of 1 was dipped in Cu(NO3)2 solution (0.05 M) in DMF at room temperature, colorless crystal of 1 transformed into a blue color crystal 1Cu after 48 h (Figure S28, Supporting Information). Although the crystals of 1Cu were poorly diffracting, we were lucky to collect the unit-cell parameters, which showed agreement with that of 1 (Table S3, Supporting Information). The energy-dispersive X-ray spectroscopy (EDS) analysis (Figure S29, Supporting Information) showed the exchange from Zn(II) to Cu(II). It was also noticed that in 1, the urotropine molecule was a dangling ligand. Therefore, replacement of the urotropine molecules by linear colinkers having donor atoms at either end should lead to a 3D pillared-layer framework. The pillar insertion reaction was probed with the dipyridyl linker azp (Scheme 1). When the colorless crystals of 1 were soaked in dark orange-colored solution of azp in DMF (0.05M) at room temperature (RT) for 3 d, color of the crystals changed to orange due to insertion of azp. Single-crystal X-ray diffraction study of the resulting orange crystal divulged its transformation 8852

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Figure 8. Constant force AFM phase image of ultrathin film of 1 in azp on HOPG (0001) surface taken at time 0 h (a, b, c), 24 h (d), 48 h (e), and 72 h (f). Rodlike features (appearing blue) and uniform domains (marked with pink dashed line) are typically observed on the surface. (inset, b) FFT of image a. The size of the FFT image is 0.081/nm. Rotational and mirror domains are indicated by red and white arrows, respectively. (inset d−f) Presence of these domains are marked with white dashed circles.

Furthermore, the structural transformation of mother crystal 1 to the daughter crystal 2 was also investigated through highresolution transmission electron microscopic (HRTEM) studies (see Supporting Information). Also, the onset of linker exchange was monitored by the progressive change of PXRD patterns (Figure 9b) of mother crystal 1 to the daughter crystal

solution on highly oriented pyrolytic graphite (HOPG) surface to understand the microscopic pattern. Figure 8a is the AFM phase image taken immediately after drop-casting. A zoom of this region shown in Figure 8b indicated ∼60% of the surface to be covered with rodlike structures aligning at different directions. A fast Fourier transform (FFT) in the inset of Figure 8b shows these rodlike structures to be following in three symmetry directions of graphite with a height of ∼0.6 nm, assigned as rotational domains. FFT further revealed that the presence of mirror domains (splitting of spots) were rotated by ±4° with respect to each other. The width of these rodlike patterns varied between 6 and 12 nm, which we assumed to be originating from molecular packing. The width of such rods suggested that each rod may consist of 3−7 molecular rows along the long growth direction. This was further confirmed in a different preparation (substrate heated to 180 °C for 30 min), where well-defined rods forming domains with inter rod distance of 6 ± 0.3 nm were observed noting that AFM was not resolving the molecular rows but possible superstructures originating from Moire-like patterns.40 The distance was 4 times intermolecular spacing along the x-axis according to the crystal structure. Along with the rodlike features, uniformly high (0.5 nm) islands were also observed (marked with pink dashed line in Figure 8a,b,e), which were less abundant compared to the other. The height of both types of islands suggested a monolayer formation. Images taken after 24 h (Figure 8d), 48 h (Figure 8e), and 72 h (Figure 8f) showed that the nature of both types of domains remained intact, and no striking change in the microscopic pattern was observed. This was also observed in the FFT (inset of d, e, f) obtained from large overview images (Figure S32, Supporting Information). From these observations, we concluded that there was no transformation of structure in the microscopic level (considering both the internal structure and growth) even after a sufficiently long time period (72 h).

Figure 9. (a) The single-crystal images of SC−SC transformations at different time intervals, (b) PXRD patterns to show the progress of linker exchange: (i) simulated 1, (ii) as-synthesized 1, and after different time exposure at (iii) 10 h, (iv) 20 h, (v) 30 h, (vi) 40 h, (vii) 50 h, (viii) 60 h, (ix) 70 h, and (x) simulated 2.

2 as explained in the Supporting Information. All our efforts to synthesize 2 via solvothermal method using different stoichiometric ratios of H2L, azp, and Zn(NO3)2·6H2O always afforded the framework {[Zn2(L)2(azp)]·8DMF·3H2O}n (3) having a different structure. The framework 3 crystallized in the triclinic space group P1̅ (Table S1, Supporting Information). Here, the carboxylate groups of L2− coordinated to Zn(II) ions 8853

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Research, New Delhi, India, to V.S., D.D., and S.P.; P. S. acknowledges DST-INSPIRE. The authors also gratefully acknowledge Prof. T. G. Gopakumar for helpful discussion.

leading to a paddle-wheel Zn2(COO)4 SBU. Each paddle-wheel unit is connected to six such units via L2− and azp ligands to form a cubane framework with twofold interpenetration (Figure S33, Supporting Information).

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CONCLUSIONS In summary, we have successfully synthesized a new porous 2D coordination network {[Zn3(L)3(urotropine)2]·2DMF·3H2O}n (1) based on [Zn3(COO)6] SBU. The exposed naphthalene moieties make 1 highly emissive and allow detection of electron-deficient nitro explosives in aqueous media, where metal-bound urotropine also plays its part. It seems promising to judiciously extend and improve the strategy adopted herein to obtain even better sensors for explosives for real-time monitoring. The axially bound urotropine molecules can take part nicely as a heterogeneous catalyst in the Baylis−Hillman reactions between MVK and substituted benzaldehydes affording satisfactory isolated yields. Moreover, the catalyst is easily recovered and recycled without loss in activity up to four cycles at least. Interestingly, compound 1 shows a heterogeneous pillar insertion reaction of azp linker to afford a 3D MOF {[Zn(L)(azp)]·4DMF·2H2O}n (2) without losing crystalinity. During this unusual 2D→3D transformation reaction, the terminal urotropine ligands were replaced by azp colinkers with concomitant change of the trinuclear [Zn3(COO)6] SBU to a mononuclear node. Additionally, framework 1 undergoes complete Cu(II) transmetalation to afford isostructural 1Cu. It appears, therefore, that the linker offers versatile ways of designing and synthesizing coordination polymers for useful practical applications.

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ASSOCIATED CONTENT

* Supporting Information S

(CIF) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.inorgchem.7b00777. Several spectroscopic, TGA, and PXRD patterns, AFM and HRTEM images, X-ray crystallographic data and figures. Crystallographic data in CIF format for 1 (CCDC 1510679), 2 (CCDC 1510680), and 3 (CCDC 1510681) (PDF) Accession Codes

CCDC 1510679−1510681 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Parimal K. Bharadwaj: 0000-0003-3347-8791 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Department of Science and Technology, New Delhi, India (to P.K.B.), and SRF from the Council of Scientific and Industrial 8854

DOI: 10.1021/acs.inorgchem.7b00777 Inorg. Chem. 2017, 56, 8847−8855

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DOI: 10.1021/acs.inorgchem.7b00777 Inorg. Chem. 2017, 56, 8847−8855